The present invention relates to an apparatus for and a method of controlling an automatic transmission, especially an automatic transmission constituted to carry out a gear change by replacing friction engagement elements.
There has been known a conventional automatic transmission constituted to control the engagement and release of friction engagement elements by the oil pressure and to carry out a gear change by making friction engagement element replacement in which an engaging control and a releasing control of two different friction engagement elements are conducted at the same time (refer to Japanese Unexamined Patent Publication 9-133205).
When a gear change is carried out by replacing friction engagement elements, as described above, if a releasing control is made too earlier than an engaging control, it generates a racing of engine rotation. On the contrary, if the releasing control is made more slowly than the engaging control, it generates a drive torque drag. Therefore, the friction engagement elements must be replaced while restricting the racing of engine rotation or the generation of drive torque drag.
Accordingly, there has been conventionally conducted a control wherein an input shaft torque of a speed change mechanism is estimated, and the sharing burden of the torque transmission is gradually transferred from a release side friction engagement element to an engage side friction engagement element, while the torque capacity capable of transmitting the input shaft torque being shared and ensured by the release side friction engagement element and the engage side friction engagement element.
Specifically, the transmission torque capacity of each friction engagement element is calculated based upon an estimation value of the input shaft torque and a torque sharing ratio changing with time (Japanese Unexamined Patent Publication 11-063201).
However, since a change velocity of the torque sharing ratio is set irrespective of an input shaft torque of a speed change mechanism, there occurs a problem in that a change amount per one step of the transmission torque capacity becomes extremely small under a situation where the input shaft torque is low, causing prolongation of the gear change period, on the other hand, the change amount per one step of the transmission torque capacity becomes too much under a situation where the input shaft torque is high, causing a gear change shock.
In view of the forgoing problems, the present invention has an object of providing an apparatus for and a method of controlling an automatic transmission capable of obtaining an appropriate step change amount of a transmission torque capacity regardless of an input shaft torque amount to avoid occurrence of prolongation of a gear change period and a gear change shock.
In order to achieve the above object, according to the present invention, in a structure wherein a transmission torque capacity of a friction engagement element at gear changing is determined based upon an input shaft torque of a speed change mechanism and a torque sharing ratio changing with time elapse, the constitution is such that a change velocity of the torque sharing ratio is altered in accordance with the input shaft torque of the speed change mechanism.
According to this constitution, a torque sharing ratio of a release side friction engagement element is reduced with time elapse and a torque sharing ratio of an engage side friction engagement element is increased with time elapse so that a transmission torque capacity of each friction engagement element is determined based upon the input shaft torque at that time and the torque sharing ratio of each friction engagement element, and a change velocity for changing the torque sharing ratio is set to be different according to the input shaft torque, to thereby adjust a step change amount of transmission torque capacity.
Here, the step change amount of the transmission torque capacity is properly set by setting the change velocity of the torque sharing ratio to be higher when the input shaft torque is smaller.
Also, the constitution may be such that the change velocity of the torque sharing ratio of when the transmission torque capacity of the release side friction engagement element is changed from a value greater than a critical transmission torque capacity toward a value smaller than the critical transmission torque capacity, is altered in accordance with the input shaft torque, and further, the alteration of the change velocity may be done only at an upshift gear change in a driving condition of an engine.
Moreover, the constitution may be such that the torque sharing ratio is determined based upon a critical torque ratio and a tolerance range changing with time elapse, and a change velocity of the tolerance range is altered in accordance with the input shaft torque of the speed change mechanism.
Further, the constitution may be such that the tolerance range is reduced from an initial value to a target value within a predetermined time so that the transmission torque capacity of the release side friction engagement element is gradually reduced from a greater value than the critical transmission torque capacity to a smaller value than the critical transmission torque capacity, to thereby alter the target value in accordance with the input shaft torque of the speed change mechanism.
The objects and features of the present invention will be understood from the following description of embodiments with reference to the accompanying drawings.